1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of effectively fixing a toner image on a recording sheet by using induction heating in a fixing unit providing improved efficiency in heating and maintenance. The present invention also relates to the fixing unit and an induction heater used therein.
2. Description of the Background Art
Background image forming apparatuses, such as copiers and printers, include fixing units using an induction heating method. The induction heating method may shorten a time period required for the fixing units to become operable after the fixing units are powered on, and may reduce an energy consumption.
One example of the fixing units includes a fixing belt, a support roller, an auxiliary fixing roller, an induction heater, and a pressure roller. The fixing belt is laid across the support roller and the auxiliary fixing roller. The induction heater faces the support roller via the fixing belt. The pressure roller faces the auxiliary fixing roller via the fixing belt. The induction heater includes an exciting coil and a core. The exciting coil is provided along the core and extends in directions parallel to a surface of a recording sheet in conveyance and perpendicular to a conveyance direction of the recording sheet which is conveyed between the pressure roller and the auxiliary fixing roller.
A high-frequency alternating current is applied to the exciting coil to generate a magnetic field around the exciting coil. The magnetic field induces an eddy current near a surface of the support roller. An electrical resistance of the support roller generates Joule heat. The Joule heat is transferred to the fixing belt from the support roller. The heated fixing belt heats and fixes a toner image on the recording sheet at a position where the pressure roller and the auxiliary fixing roller oppose to each other.
In the above fixing unit, it is possible to increase a surface temperature of the fixing belt to a target fixing temperature in a short time period without consuming much energy. However, the exciting coil may face the support roller at variable positions, resulting in a fluctuation of the heating efficiency.
Another example of fixing units includes a fixing roller and an exciting coil. The fixing roller includes a hollow cylinder. The exciting coil includes a wire wound around the fixing roller a plurality of times, so that the exciting coil faces outer and inner circumferential surfaces of the fixing roller.
In the above fixing unit, it is possible to effectively increase a surface temperature of the fixing roller. However, once the wire is wound around the fixing roller, the wire cannot be separated from the fixing roller, resulting in inefficient maintenance operations.
In the above fixing units, a temperature of a part of the fixing belt or the fixing roller may overly increase when the image fixing is conducted a number of times in a consecutive manner relative to smaller-size recording sheets. A temperature of the whole fixing belt or fixing roller may overly increase when the fixing unit accidentally stops operating due to a paper jam.